Switching power supply devices including semiconductor devices have been widely used for household equipment, such as electric appliances, as its power supply devices in order to reduce its electric consumption and improve its power efficiency. The semiconductor devices utilize a switching operation performed by semiconductor (a switching element, such as a transistor) to control (to stabilize) an output voltage.
Recently, in particular, global warming prevention measures have drawn a significant attention to the reduction of the stand-by electricity consumed by electric devices, such as the household appliances. This trend generates a great demand for a switching power supply device consuming less stand-by electricity.
In most cases, a typical energy loss of a switching power supply under the light load, such as a stand-by load, is due to a switching loss caused by a switching operation. One of the techniques to improve power efficiency under the light load, such as the stand-by load, is to employ intermittent oscillation control which intermittently carries out a switching operation under the light load.
FIG. 16 depicts a functional block diagram exemplifying a structure of a switching power supply device 900 including a control circuit 901 which carries out conventional intermittent oscillation. Briefly described below is a switching operation by the intermittent oscillation control in the switching power supply device 900, with reference to a timing diagram in FIG. 17.
In the switching power supply device 900 shown in FIG. 16, the switching operation is carried out via quasi-resonant control (also referred to as bottom-on control) in a current mode. In the quasi-resonant control in a current mode, a switching element 2 turns OFF when a current ID flowing in the switching element 2 reaches its target value, and turns ON at the bottom; that is, the local minimum point of a ringing voltage which develops in the switching element 2 when the switching element is OFF. The bottom of the ringing voltage is detected by a bottom detecting circuit 17, and is indicated by a signal Bottom outputted from the bottom detecting circuit 17.
In the load varying state in FIG. 17, an output voltage Vout increases as an output current Iout gets out of the load rated state and decreases. A feedback signal IFB (for example, a current signal which flows out of the control circuit, and increases as the output voltage Vout increases) indicating the magnitude of the output voltage Vout is provided from an output voltage detecting circuit 6 to an FB terminal. As the output voltage Vout is greater, a feedback control circuit 12 outputs, based on the feedback signal IFB, a control signal VEAO indicating a smaller limiting value with respect to a current ID flowing in a switching element 2.
When the load becomes even smaller, an intermittent oscillation control circuit 18 brings a signal Enable low so that the low signal Enable indicates a suspension of the switching operation. Once the signal Enable goes low, the signal Bottom is cut by a turn-on control circuit 93. This operation suspends the switching operation in the switching element 2.
There is an edge at which the signal Enable goes low to indicate the suspension of the switching operation. This edge is also referred to as an intermittent suspension signal. Suspension of the switching operation caused by the intermittent oscillation control is also referred to as intermittent suspension.
As the switching operation is kept suspended, the output voltage Vout decreases. Thus, the intermittent oscillation control circuit 18 brings the signal Enable high so that the high signal Enable indicates execution of the switching operation. An intermittent resuming circuit 91 outputs a signal Up, which is a one-shot pulse, at a rising edge of the signal Enable. The signal Up forces the switching element 2 to turn ON. Then, the switching operation is executed via the quasi-resonant control that turns ON the switching element 2 based on the signal Bottom from the bottom detecting circuit 17.
There is an edge at which the signal Enable goes high so that the high signal Enable indicates execution of the switching operation. This edge is also referred to as an intermittent resuming signal. Resumption of the switching operation via the intermittent oscillation control is also referred to as intermittent resumption.
Thus, in a first stand-by state in FIG. 17, the intermittent oscillation control is carried out to alternatively suspend and execute the switching operation on the switching element 2.
When the output current Iout becomes even smaller than that in the first stand-by state, the first stand-by state goes to a second stand-by state. In the second stand-by state, a suspension period of the switching operation is longer than that in the first stand-by state. In other words, as the load becomes lighter, an intermittent control cycle is made longer. Here, the intermittent control cycle includes an executing period and a suspension period of the switching operation on the switching element 2. Hence, the intermittent oscillation control executed under the light load improves power efficiency under the light load.
The above conventional technique, however, causes the following problem: When the signal Enable goes high, the intermittent resuming circuit 91 outputs the signal Up in order to resume the switching operation in the intermittent oscillation control, regardless of the ringing voltage developed in the switching element 2. Thus, in the worst case, the switching element 2 can turn ON at the top; that is, the local maximum point of the ringing voltage. This problem leads to a power loss in the switching element 2.
For example, suppose C is the sum of a parasitic capacitance of the switching element 2 and a capacitance of a capacitor 31 connected in parallel between the input and the output of the switching element 2, and V is a voltage when the switching element 2 turns ON. Here, when the intermittent resumption occurs, the power loss occurring in the switching element 2 is obtained as follows:[Math. 1]½CV2  (Expression 1)
The power loss is also referred to as a power loss by the capacitance C or a loss by the capacitance between the input and the output of the switching element.
In other words, the intermittent oscillation control under the light load is effective for the improvement in power efficiency under the light load. However, in the case where a voltage is high when the switching element 2 turns ON in the intermittent resumption, the capacitance C inevitably causes a large power loss as a result. In particular, when the intermittent control cycle is made shorter depending on the state of a load, the power loss becomes greater.
Patent Literatures 1 and 2, for example, disclose switching power supply devices to reduce a power loss which can be developed by the capacitance C in the intermittent resumption.
FIG. 18 depicts a functional block diagram exemplifying a structure of a conventional switching power supply device 910 which reduces a power loss that can occur in the intermittent resumption. The switching power supply device 910 is a modified version of the switching power supply device 900 in FIG. 16 based on an idea disclosed in Patent Literatures 1 and 2. In a control circuit 911, an intermittent resuming circuit 92 replaces the intermittent resuming circuit 91.
Described hereinafter is how the switching power supply device 910 operates in accordance with control techniques in Patent Literatures 1 and 2, with reference to timing diagrams in (a) and (b) in FIG. 19.
In the switching power supply device 910, the intermittent resuming circuit 92 times a predetermined waiting time period t (transformer reset detection time in Patent Literatures 1 and 2). Here, the time period t starts at an intermittent suspending signal (a falling edge of the signal Enable) as the starting point of the timing. The intermittent resuming circuit 92 outputs a low signal Mask in order to indicate that the intermittent resuming circuit 92 is timing the waiting time period t. Then, in the case where an intermittent resuming signal (the rising edge of the signal Enable) arrives before the intermittent resuming circuit 92 finishes timing the waiting time period t, a turn-on control circuit 94 outputs as a signal TurnOn a signal Bottom (the transformer reset signal in Patent Literatures 1 and 2) to be provided after the intermittent resumption, and turns ON the switching element 2 (the illustration (a) in FIG. 19).
In the case where an intermittent resuming signal (the rising edge of the signal Enable) does not arrive before the intermittent resuming circuit 92 finishes timing the waiting time period t, the intermittent resuming circuit 92 outputs, upon arriving the following intermittent resuming signal, a signal Up (the intermittent end pulse in Patent Literatures 1 and 2) which is a one-shot pulse. The turn-on control circuit 94 outputs the signal Up as a signal TurnOn in order to turn ON the switching element 2 (the illustration (b) in FIG. 19).
As a specific circuit to time the waiting time period t, Patent Literature 1 discloses a time constant circuit including a constant current source and a capacitor, and Patent Literature 2 discloses a counter circuit to count how many bottoms are observed in a ringing voltage.
Performing the above control, the switching power supply device 910 implements intermittent resumption at a bottom of the ringing voltage observed in the switching element, in the case where an intermittent resuming signal arrives within the waiting time when an intermittent suspending signal is designated as the starting point of the timing. Consequently, in the intermittent resumption, the switching power supply device 910 can reduce a loss caused by a capacitance between the input and the output of the switching element.